Monroe County, New York, named for President James Monroe, was created from portions of Ontario and Genesee Counties on February 23, 1821. The county already included 28,000 settlers in growing towns and villages that provided agricultural products and market support for the burgeoning mill town known as Rochester.

When counties were established in the Province of New York in 1683, the present Monroe County was part of Albany County. This was an enormous county, including the northern part of New York State as well as all of the present State of Vermont and, in theory, extending westward to the Pacific Ocean.